Antireflection coatings are employed on photodetecting materials in order to minimize losses due to reflection of incident radiation at the material/air boundary. As can be appreciated, any incident radiation which is reflected from the boundary is unavailable to be absorbed within the material and is therefore undetectable. This problem is especially apparent with modern photodetecting devices, such as photodetectors comprised of Group II-VI material, which are designed to detect relatively small amounts of incident infrared (IR) radiation. The photodetecting device may have a substrate comprised of an alloy such as cadmium-zinc-telluride (CdZnTe) or cadmium-telluride (CdTe) and an overlying epitaxial radiation absorbing layer of mercury-cadmium-telluride (HgCdTe). Such a device may be fabricated as a backside illuminated photodetector wherein the incident radiation enters the photodetector through a back surface of the transparent CdZnTe substrate layer, the back surface being opposite a surface wherein an array of photodiodes or photoconductors is formed within the HgCdTe epilayer.
An antireflection coating preferably has an index of refraction which provides the desired antireflection characteristics while also exhibiting the property of being substantially transparent to the incident radiation. The coating should also be amenable to being formed at a relatively low temperature to avoid altering the stoichiometry of the HgCdTe layer. The coating should also be durable and be capable of withstanding typical photodetector fabrication techniques such as thermal cycling, solvent rinsing, acid rinsing and similar processes.
Until now, no antireflection coating was known for CdTe or CdZnTe which exhibited the desirable qualities set forth above.
It is therefore one object of the invention to provide an AR coating material having an index of refraction which causes minimum reflection from a CdZnTe substrate or other substrate materials having refractive indices in the range between approximately 2.2 and approximately 2.8.
It is another object of the invention to provide an AR coating which causes minimal reflection of radiation within a range of wavelengths between about 1 and 12 micrometers and which also exhibits little or no absorption at wavelengths between about 1 and 9 micrometers.
It is another object of the invention to provide an AR coating which adheres durably to CdZnTe during the chemical and handling processes experienced in the fabrication of detectors and which further may be applied early in the detector device fabrication process, thereby increasing device production yield.
It is another object of the invention to provide an AR coating which increases the detective quantum efficiency of backside illuminated detectors.
It is a further object of the invention to provide an AR coating which exhibits sufficiently low mechanical stress to permit the coating to be grown to physical thicknesses corresponding to quarter wavelength optical thicknesses at long infrared wavelengths.
It is one further object of the invention to provide an AR coating which is comprised of a fluoride compound formed with a metal of the Lanthanide rare earth series which is transparent to infrared radiation.
It is another object of the invention to provide an AR coating which is deposited by a process of low temperature, high energy ion beam sputtering of a CeF.sub.3 source material.
It is another object of the invention to provide an AR coating process in which a small quantity of oxygen is introduced to a growing CeF.sub.3 film to serve as a substitute for fluorine atoms which are partially depleted during the sputter deposition of the CeF.sub.3 source material and thus restore the transparency which is typically decreased with the loss of flourine atoms.
It is another object of the invention to provide an AR coating process whereby the resulting CeO.sub.x F.sub.(3-x) layer exhibits low radiation absorption and a corresponding high transmissibility due to the substitution of oxygen atoms for depleted fluorine atoms.
It is a still further object of the invention to provide a low temperature AR coating process that is suitable for HgCdTe detector fabrication without risk to the stoichiometry of the HgCdTe.